Update types to not allow undefined on store state

Update types to not allow undefined on store state.
Currently using undefined as the store initial value leads to the initial value being an empty object, which is not correct.
So by restricting the type to not be undefined, users can just use null and that will work fine.

types/index.d.ts

@@ -1,4 +1,6 @@
 declare module 'react-hookstore' {
+  type NonUndefined<T> = T extends undefined ? never : T;
+
   type StateCallback<TState> = (state: TState) => void;
 
   type ReducerType<TState, TPayload = any> = (state: TState, payload: TPayload) => TState;
@@ -32,11 +34,11 @@ declare module 'react-hookstore' {
       dispatch<TPayload>(payload: TPayload, callback?: StateCallback<TState>): void;
   }
 
-  export function createStore<TState, TPayload = any>(name: string, state: TState, reducer: ReducerType<TState, TPayload>): ReducerStoreInterface<TState, TPayload>;
+  export function createStore<TState, TPayload = any>(name: string, state: NonUndefined<TState>, reducer: ReducerType<TState, TPayload>): ReducerStoreInterface<TState, TPayload>;
 
-  export function createStore<TState>(name: string, state: TState): StateStoreInterface<TState>;
+  export function createStore<TState>(name: string, state: NonUndefined<TState>): StateStoreInterface<TState>;
 
-  export function createStore<TState>(name: string, state: TState, reducer: ReducerType<TState>): ReducerStoreInterface<TState>;
+  export function createStore<TState>(name: string, state: NonUndefined<TState>, reducer: ReducerType<TState>): ReducerStoreInterface<TState>;
 
   export function getStoreByName<TState, TPayload = any>(name: string): StateStoreInterface<TState> | ReducerStoreInterface<TState>;